Re: How to achieve a long burst?

[Neal Palmer <neal@dinigroup.com>]

Another reason that people don't like to have the CPU push large chunks of
data to the card is that it basically stops the processor from doing other
useful work while it waits for the PCI bus to accept the data.

But if you do a DMA, then the processor needs to do about 4-5 writes, and
then can do some other task (like running a user application) while the
large chunk of data transfers.  Also it makes it easier to manage having
multiple cards in the computer all transferring data at the same time (and
having lower latencies for IDE/SCSI and NIC devices).

On Thu, 14 Dec 2000, Henry Gong wrote:

> 
> Dear all,
> 
> 
> There is a popular view that if you want a long burst, you have to be a
> bus master.  It sounds reasonable but nobody can tell me why. Can
> anybody explain please?
> 
> If the PC has a large block of data to send to the PCI card, here is
> what an experienced PCI engineer likes to do:
>     1. The host sends a message to the PCI card informing the starting
> address;
>     2. The PCI card starts a "Mater Read".
> 
> In this way, the transfeer is done in two steps. This is certainly not
> efficient for short bursts. It is supposed to be good for long burst.
> But why cannot the host simply start a master write. Then the PCI card
> responds in "Slave Read"? People beleive that the host does not like to
> use long
> bursts. It normaly separates a long burst into many short bursts. Is
> this true? Why?
> 
> If the host sends a block of data to a PCI card, how long burst in data
> can it
> normally achieve, 20, 200 or 2000?
> 
> If a PCI card sends a block of data to the host, how long burst in data
> can it
> normally achieve, 20, 200 or 2000?
> 
> How can device driver control the burst? For example, why do people say
> 3D graphic normally uses many short bursts?
> 
> Sorry to ask so many questions. Any explanation and comment, even blame,
> is appreciated.

-- Neal Palmer

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